1. Field of the Invention
The invention concerns welding apparatus, particularly for inert-gas DC welding, with an adjustable operating current, which for the purpose of avoiding end craters at welded connections has an automatic controller for continuously decreasing the current from the operating value--for instance, along an exponentially falling curve with adjustable time constants--and for automatic reduction from this intermediate current value to a minimum, preferably to zero, wherein these reductions can be initiated by a stepping circuit actuated preferably by one single control contact. Such apparatus has been described and claimed in DE-OS No. P2740529.4(=British Patent Application No. 39379/77), hereafter referred to as "the earlier invention".
2. Description of the Prior Art
In such types of welding apparatus, an increase or decrease of the operating current, hereafter designated "the welding current", is effected by pressing or releasing a switch on the welding torch. The control steps or stages, which are characterised mainly by the magnitude of the current, are controlled by a stepping circuit or switch serving automatically to effect stepping from one control state to the next. According to the respective switch-off times of the whole welding apparatus, the contacts of the current impulse switch of the stepping circuit are either closed or open. According to the position of the current impulse switch, when the supply voltage is switched on again the stepping circuit is in a stationary position before the first actuation of the torch switch or a control stage of the welding process. Consequently after the supply voltage is re-established the welding electrode passes voltage to the workpiece to be welded. On the first actuation of the torch switch the stepping circuit is accordingly either in its first control stage wherein arc striking and locating (seeking) takes place, or in the third control stage (reduced current strength) of releasing and holding. On releasing the torch switch the stepping circuit then automatically steps to the second control stage (welding) or to the fourth control stage (reduction or drop-out). For an operator working remotely from the apparatus it is not or barely possible to discern on actuation of the torch switch which control stage the welding apparatus is actually in.
Further, in the welding apparatus according to the earlier invention the adjustment of the magnitude of the crater filling current takes place independently of the magnitude of the welding current. Accordingly, each time the welding current is changed the crater filling current must also be re-adjusted so that welding is always effected with a constant ratio of crater filling current to welding current. The magnitude of the welding current corresponds to the magnitude of the crater filling current in the first control stage (striking and seeking). In the first control stage it is not possible to adjust the welding current independently of the crater filling current.
In the welding apparatus according to the earlier invention the transition from the control stage of welding to the control stage of releasing and holding as well as from the last-mentioned control stage to the control stage of drop-out is always regulated to a presettable constant intermediate current value along an exponentially falling curve with adjustable time constants. It is not possible so to exercise control as to perform the transition from one control stage to the next by a more or less sudden jump of predetermined magnitude.